Steel corrosion in reinforced alkali-activated materials

Abstract

The development of alkali-activated materials (AAMs) as an alternative to Portland cement (PC) has seen significant progress in the past decades. However, there still remains significant uncertainty regarding their long term performance when used in steel-reinforced structures. The durability of AAMs in such applications depends strongly on the corrosion behaviour of the embedded steel reinforcement, and the experimental data in the literature are limited and in some cases inconsistent. This letter elucidates the role of the chemistry of AAMs on the mechanisms governing passivation and chloride-induced corrosion of the steel reinforcement, to bring a better understanding of the durability of AAM structures exposed to chloride. The corrosion of the steel reinforcement in AAMs differs significantly from observations in PC; the onset of pitting (or the chloride 'threshold' value) depends strongly on the alkalinity, and the redox environment, of these binders. Classifications or standards used to assess the severity of steel corrosion in PC appear not to be directly applicable to AAMs due to important differences in pore solution chemistry and phase assemblage.

Chloride‐induced steel corrosion in alkali‐activated fly ash mortar: Increased propensity for corrosion initiation at defects

“…These values are in line with previous observations and indicate that in both materials, alkali‐activated fly ash mortar and Portland cement mortar, the steel had attained the passive state during curing. [ 9,21 ] The ohmic resistances ( R el ) after curing were 500–600 Ω for the Portland cement mortar specimens and ~400 Ω for the alkali‐activated fly ash mortar (Figures S1–S4), indicating a finer pore structure and possibly a lower ionic strength of the pore solution in the Portland cement mortar.…”

“…While extensive data regarding the critical chloride content in concretes and mortars based on Portland cement and blended cement have been compiled, [ 6–8 ] much less data in this regard are available for concretes or mortars based on alkali‐activated binders. [ 9 ] Early work showed that carbon steel in chloride‐free alkali‐activated fly ash mortars attains a passive state and indicated that chloride contents up to 0.4 wt% with respect to binder (wrtb) do not cause steel corrosion in these mortars. [ 10 ] The authors of a more recent study [ 11 ] concluded that the critical chloride content for the onset of steel corrosion in alkali‐activated fly ash mortars is in the range 1–1.7 wt% wrtb.…”

“…[ 15 ] In addition, recent work has highlighted the importance of the redox conditions in the pore solution of alkali‐activated binders, which is mainly controlled by sulfides from slag in the binder, for chloride‐induced corrosion of embedded steel. [ 9,16 ]…”

Alkali Activated Slag Mortars Provide High Resistance to Chloride-Induced Corrosion of Steel

“…For BFS and WPS mortars, the sulfide supplied by the slag was released and altered the passive film into a sulfidic layer that has a different electrochemical response (Mundra et al, 2017a). Therefore, the combination (R f -CPE f ) was not observed, except when they were exposed to standard laboratory conditions or when WPS mortar was immersed in the alkaline solution at early ages.…”

“…During the hydration of PC, portlandite [Ca(OH) 2 ] forms as an important hydration product, and plays important role in delaying the onset of the corrosion of embedded steel, as it can maintain the high pH of the cement pore solution through a buffering mechanism (Bertolini et al, 2013). However, in AAMs portlandite is not typically identified as a reaction product (Lloyd et al, 2010), and therefore, the availability (or conversely, leaching) of OH − ions from the pore solution at the steel-concrete interface will have a greater influence in ensuring the stability of this protective layer in AAMs (Mundra et al, 2017d).…”